Tube waves (or Stonely waves) are plane pressure waves that propagate through a tubular medium including annuli. These waves reflect from changes in the characteristic impedance of the medium. Examples of characteristic impedance changes include: a pipe diameter change, a closed end, a free surface, a gas bubble, a compressibility or density variation, a fluid change causing a change in the speed of sound, a pipe elastic modulus change, holes in a tubular with flow capacity, and so on. Combined with some knowledge of the wellbore geometry and/or the speed of the tube wave, the complex reflection patterns can be interpreted to yield useful information about the wellbore. Exemplary usages include locating the top of cement, identifying the setting of cement, locating which perforations in a well are passing fluid, confirming shifting of control valves, locating coiled tubing relative to downhole features, and so on.
Uniformly generated tube wave reflections, for example from tube waves generated by a constant frequency source, can be difficult to identify in noisy well pumping situations, for example during hydraulic fracturing or other treatments. Further, the penetration depth of a tube wave into a tube is inversely related to the frequency of the generated tube wave. Conversely, the resolution of the tube wave technique is directly related to the frequency of the generated tube wave. Accordingly, detection of various features in a wellbore may be amenable to various detection frequencies. Additionally, high energy tube waves are easier to detect than lower energy tube waves. State of the art impulsive pulse generators operate at less than about 3,500 kPa pulse amplitude and deliver pulse energy of less than about 1,000 joule.